Problems with Testing CEL

[drwambo]

Hi Folks,

Have had some rough running issues while the engine is hot, so I took my truck to my local mechanic. He used a Snap On CEL reader to test the truck. It cam back with a bad upstream O2 sensor and an incomplete test message. Replaced the sensor, and the same error still comes up. Trying to find out 2 things:

1) why would my brand new O2 sensor still show a fault - it's apparently good.

2) Why would the computer not complete the testing.

The incomplete test message shows up on both running and not running tests. Do I need a new computer? Engine is completely stock except for K&N filter and removed intake cover from wheel well to get more air in the the engine.

Any thoughts would be greatly appreciated..




Douglas

 

[aldive]

Strange one.

Have you had a scan done using another scanner? It could be a faulty scanner.

Keep us posted.... good luck.

 

[drwambo]

Only used his tester, but it works on other vehicles. Kinda' worried that my computer isn't working properly.....
Hate like heck to take it to the dealer.....

 

[MrShorty]

Had an old Celebrity that showed an O2 sensor fault (indicating rich). The fault remained after replacing the sensor. I took it to a shop to get a more thorough diagnosis. He hooked it up to his scanner and after a round of "it needs a new O2 sensor" "it already has a new O2 sensor, try again" He decided it needed a new computer, which fixed the problem. Diagnosis went something like this: O2 is showing rich. While monitoring O2 sensor output at computer, force lean condition by causing a vacuum leak. O2 sensor continued to show rich. After doing variations on this theme all day, he came back to me and said "by process of elimination, I think the computer is bad."
Just one case where it was a bad computer. Before you jump out and replace the computer, I'd try some more diagnosis, maybe from someone with a better scanner, or who knows how to get more information from the scanner. Also, if you haven't already, be sure the wiring from the sensor to the computer is good.

 

[Robert]

Just to let you know, Ford did some weird stuff with their OBD-II and oxygen sensor tests when they first switched over to OBD-II so it may not be your computer. I know my scanner will not perform O2 tests on a '97 or '98 Explorer. The manufacturer of my scanner is aware of the problem so they let me borrow their $2000 professional scanner for a week to troubleshoot my Explorer. Their professional grade scanner had no problems running O2 tests. Over the last couple of years, my scanner has gone through numerous software updates. I can now monitor my fuel trims and O2 output voltages using the Ford Enhanced OBD-II support wheras the Standard Ford mode (IE Generic OBD-II) still does not support the O2 tests. I am not familiar with the Snap-on scanner, but I would think it would be a professional grade scanner though. I would still try a different brand to be sure before commiting to a new computer.